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Quantitative analysis of amino acids is performed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) by labeling amino acids present in plasma, urine, and cerebrospinal fluid with aTRAQ Reagent 121. Samples are dried and reconstituted with aTRAQ Reagent 113-labeled standard mix. Amino acids are separated and detected by LC-MSMS (API 3200). The concentrations of amino acids are established by comparison of their ion intensity (121-labeled amino acids) to that of their respective internal standards (113-labeled amino acids). Chromatography is performed using a C18 (150 x 4.6-mm) column and total analysis time is 18 minutes.(Unpublished Mayo method)